Demand for secondary batteries, such as Ni-MH secondary batteries or lithium secondary batteries, has been increased as development of portable devices, such as mobile phones, notebook computers, and camcorders, has increased. In particular, since lithium secondary batteries using lithium and non-aqueous electrolyte solution are more likely to be realized as compact, lightweight, and high energy density batteries, development of the lithium secondary batteries has been actively conducted. In general, transition metal oxides, such as LiCoO2, LiNiO2, and LiMn2O4, are used as a positive electrode material of a lithium secondary battery, lithium metal or carbon is used as a negative electrode material, and an organic solvent containing lithium ions is used as an electrolyte between two electrodes to constitute a lithium secondary battery.
However, with respect to a lithium secondary battery using the lithium metal as a negative electrode, since dendrites may be easily generated when charge and discharge are repeated and, as a result, the lithium secondary battery may have high risk of short circuits, a lithium secondary battery has been commercialized in which a carbonized or graphitized carbon material is used in the negative electrode and a non-aqueous solvent containing lithium ions is used as an electrolyte. However, since a carbon-based negative electrode material has large irreversible capacity, its initial charge and discharge efficiency may be low and capacity may be reduced. In addition, since lithium may be precipitated on the surface of carbon during overcharging, it may cause problems in safety.
A lithium titanium oxide, which is recently in the spotlight as a negative electrode material of a lithium ion battery, may have limitations in that an operating voltage is high at 1.3 V to 1.6 V in comparison to a typical carbon-based negative electrode material and irreversible capacity is low at about 170 mAh/g, but may achieve excellent safety, because high-speed charge and discharge is possible, an irreversible reaction almost does not occur (initial efficiency of 95% or more), and the heat of reaction is very low. Also, with respect to the carbon material, theoretical density is low at about 2 g/cm3, but, since Li4Ti5O12, as one type of the lithium titanium oxides, has a high theoretical density of about 3.5 g/cm3, capacity per volume is similar to that of the carbon material.
When an electrode is realized by actually using the lithium titanium oxide as an active material, it is an important task to increase the capacity per volume by increasing the density of the electrode while maintaining rate capability as high as possible in the synthesis of the lithium titanium oxide.
Also, with respect to a lithium secondary battery using the lithium titanium oxide as a negative electrode, there is a limitation in that gas is generated by lithium remained in the lithium titanium oxide when stored for a long period of time at high temperature. When the gas is generated in the battery, there is a risk of causing a serious problem in safety, for example, explosion due to the expansion of the battery, and thus, there is a need to develop a lithium titanium oxide-based negative electrode material in which the expansion of the battery does not occur due to a small amount of the generated gas while maintaining high rate capability.